This invention relates to wetting resistant materials. More particularly, this invention relates to articles that include coatings of wetting resistant materials.
The “liquid wettability”, or “wettability,” of a solid surface is determined by observing the nature of the interaction occurring between the surface and a drop of a given liquid disposed on the surface. A high degree of wetting results in a relatively low solid-liquid contact angle and large areas of liquid-solid contact; this state is desirable in applications where a considerable amount of interaction between the two surfaces is beneficial, such as, for example, adhesive and coating applications. By way of example, so-called “hydrophilic” materials have relatively high wettability in the presence of water, resulting in a high degree of “sheeting” of the water over the solid surface. Conversely, for applications requiring low solid-liquid interaction, the wettability is generally kept as low as possible in order to promote the formation of liquid drops having high contact angle and thus minimal contact area with the solid surface. “Hydrophobic” materials have relatively low water wettability (contact angle generally at or above 90 degrees); so-called “superhydrophobic” materials (often described as having a contact angle greater than 120 degrees) have even lower water wettability, where the liquid forms nearly spherical drops that in many cases easily roll off of the surface at the slightest disturbance.
Heat transfer equipment, such as condensers, provide one example of an application where the maintenance of surface water as droplets rather than as a film is important. Two alternate mechanisms may govern a condensation process. In most cases, the condensing liquid (“condensate”) forms a film covering the entire surface; this mechanism is known as filmwise condensation. The film provides a considerable resistance to heat transfer between the vapor and the surface, and this resistance increases as the film thickness increases. In other cases, the condensate forms as drops on the surface, which grow on the surface, coalesce with other drops, and are shed from the surface under the action of gravity or aerodynamic forces, leaving freshly exposed surface upon which new drops may form. This so-called “dropwise” condensation results in considerably higher heat transfer rates than filmwise condensation, but dropwise condensation is generally an unstable condition that often becomes replaced by filmwise condensation over time. Efforts to stabilize and promote dropwise condensation over filmwise condensation as a heat transfer mechanism in practical systems have often required the incorporation of additives to the condensing medium to reduce the tendency of the condensate to wet (i.e., form a film on) the surface, or the use of low-surface energy polymer films applied to the surface to reduce film formation. These approaches have drawbacks in that the use of additives may not be practical in many applications, and the use of polymer films may insert significant thermal resistance between the surface and the vapor. Polymer films may also suffer from low adhesion and durability in many aggressive industrial environments.
Texturing or roughening the surface can change the contact angle of water on a surface. A texture that increases the tortuosity of the surface but maintains the contact between water droplet and the surface will increase the contact angle of a hydrophobic material and decrease the contact angel of a hydrophilic material. In contrast, if a texture is imparted that maintains regions of air beneath a water droplet, the surface will become more hydrophobic. Even an intrinsically hydrophilic surface can exhibit hydrophobic behavior if the surface is textured to maintain a sufficiently high fraction of air beneath the water drop. However, for applications requiring highly hydrophobic or superhydrophobic behavior, it is generally more desirable in practice to texture a hydrophobic surface than to texture a hydrophilic surface. An intrinsically hydrophobic surface usually provides the potential for a higher effective contact angle after texturing than an intrinsically hydrophilic surface, and generally provides for a higher level of wetting resistance even if the surface texturing becomes less effective over time as the texture wears away.
Relatively little is known about the intrinsic hydrophobicity of broad classes of materials. In general, most of the materials known to have a contact angle with water of greater than 90 degrees are polymers such as tetrafluoroethylene, silanes, waxes, polyethylene, and propylene. Unfortunately, polymers have limitations in temperature and durability that can limit their application, because many practical surfaces that would benefit from low wettability properties are subject in service to high temperatures, erosion, or harsh chemicals.
Ceramic materials are typically superior to polymers in many aspects related to durability. Of the ceramic materials, oxide ceramics are particularly useful because they are highly manufacturable, often have high environmental resistance, and can have good mechanical properties. Unfortunately, there are virtually no known oxide ceramics that are hydrophobic. A notable exception is silicalite, a zeolitic polymorph of SiO2 [E. M. Flanigen, J. M. Bennett, R. W. Grose, J. P. Cohen, R. L. Patton, R. M. Kirchner, and J. V. Smith, “Silicalite, a new hydrophobic crystalline silica molecular sieve,” Nature, v.271, 512 (1978)]. For that material the specific crystal structure is highly important because amorphous SiO2 has a very low, hydrophilic wetting angle. However, the synthesis conditions required to form zeolite crystals can limit the range of applicability of those materials as hydrophobic surfaces and the porosity of zeolite crystals makes them less desirable for applications requiring durability.
Therefore, there remains a need in the art for oxide ceramics that have lower liquid wettability than conventional oxides, promote stable dropwise condensation, are stable at elevated temperatures, are amenable to coating processing, and have good mechanical properties. There is also a need for articles coated with these wetting resistant oxide ceramics.